New iodine laser achieves positive gain
The chemical oxygen-iodine laser (COIL) is a continuous-wave, near-infrared chemical laser. It offers excellent properties for applications that require high-power laser output from a mobile platform. The best-known example of a COIL system may be the ‘Airborne Laser’ ballistic missile defense weapon system,1 which uses a multi-megawatt laser with good beam quality. For very-high-power applications, chemical lasers retain the advantages of scalability and beam quality over solid-state and fiber lasers. Therefore, intensive studies to develop the technology are being conducted internationally.
A typical COIL depends on energy transfer from singlet oxygen O2(1Δ) to the iodine atom, which is the lasing species. The O2(1Δ) is typically generated by the following chemical reaction:
This reaction illustrates an inherent drawback for the laser for mobile systems: the energy source of COIL, basic hydrogen peroxide (BHP) aqueous solution, is bulky to carry. It also decomposes over time, which limits both long-term storage of BHP and applications in remote places.
Because iodine is an ideal lasing species, scientists have sought an alternative donor that is compatible with singlet oxygen but does not depend on wet chemistry. In 2000, Gordon Hager's group at the Air Force Research Laboratory succeeded in operating an all-gas-phase iodine laser (AGIL).2 The energy donor of AGIL is an excited nitrogen chloride (NCl) molecule in the singlet state. Generating NCl(1Δ) is achieved by the following chain of gas-phase reactions:
While this reaction avoids BHP, it still requires highly toxic and explosive hydrogen azide, highly corrosive fluorine, and expensive deuterium chloride (DCl). In search of a more easily managed iodine reaction, researchers—including our group—are studying an alternative reaction called amine-based AGIL.
We have been studying a gaseous reaction that provides an alternative way of generating NCl(1Δ). Decomposition of nitrogen trichloride (NCl3) produces NCl(1Δ) with high yield by the following reaction,
and this could be used as an energy source of the AGIL.3
We built an amine-AGIL apparatus (see Figure 1). The amine AGIL is composed of a simple mixing tube and a diverging flow duct equipped with transparent windows. Three gasses, namely, NCl3, HI, and atomic H are injected into the mixing tube. HI is the iodine source. Atomic hydrogen is provided when the microwave discharge by the magnetron dissociates the hydrogen molecules.
An amine-based AGIL has been studied elsewhere,4,5 but neither lasing nor positive gain was reported. We developed a numerical simulation of the NCl3+HI+H2/H reaction system,6 and found that the injection order of the three gas species is the key to achieving positive gain. Other parameters that can be optimized include the flow rates of the gasses, the operating pressure, and the position of the optical axis.
Figure 2 shows our amine AGIL in operation. The blue glow is fluorescence in the glass tube caused by the hydrogen plasma, and the red emission in the flow duct section is the fluorescence from the NCl(B-X) transition. Pressure in the flow duct is 7T, and the flow velocity is approximately 100m/s.
We probed the small signal gain of the I(2P1/2)−I(2P3/2) transition at 1315nm by a wavelength-scanned diode laser. The optical path length of the probe beam was 8cm, and the beam passed through the duct twice. Due to floor vibrations from the vacuum system and the very small (on the order of 0.1%) round trip gain/loss, the signal-to-noise ratio was low. Nevertheless, we could see the clear difference between the gain trace with and without the microwave discharge (see Figure 3).
Our microwave generator came from a commercial microwave oven, and therefore worked for only half the cycle of the alternate current. The blue line in Figure 3 shows the absorption when the microwave is off. Iodine atoms are probably present by the following reaction,
but not pumped. When the microwave is ‘on,’ the trace turns from negative to positive, which is the proof of the following energy transition,
The round-trip small signal gain is not yet large enough to achieve lasing, but we are optimistic that we can improve it by a factor of two or three. According to the simulation, the current performance of the apparatus is limited by an insufficient concentration of hydrogen atoms. This is primarily due to the poor coupling of the microwave power to the flowing gas. We believe we are close to the first laser oscillation of an amine-based AGIL.
The authors are grateful to Kawasaki Heavy Industries Ltd. for the lease of their vacuum system and measurement devices. This work is supported by the Society of Iodine Science.
Masamori Endo is an associate professor. He has been involved in chemical oxygen-iodine lasers since 1997. He invented a novel supersonic mixing nozzle for COIL. He is also interested in optical resonator problems.